This invention is generally related to circuit interruption, and, more particularly, this invention is related to detecting series arcing in an arc fault circuit interrupter.
Arcing is the luminous discharge of electricity across an insulating medium, usually accompanied by the partial volatilization of the electrodes. Arcing faults in ac electrical systems are of two types: parallel arcs and series arcs. In a parallel arcing fault, the arc occurs across two conductors in the branch circuit or extension cords plugged into it, at a site where the insulating media separating the two conductors has been compromised. The arc may occur across the line and neutral conductors or the line and ground conductors, or in the case of reverse polarity where the line voltage is reverse polarized, between the neutral and ground conductors. The current through the parallel arc fault is not limited by the impedance of the load, but rather by the available current from the supply established by the impedance of the conductors and terminals between the source of line voltage and the position of the parallel fault, that is, the conductive members carrying the fault current. Parallel arcing faults can draw considerable current but usually below the pick-up current of the typical circuit breaker. They also tend to be intermittent because of the repulsion forces generated by the current of the arc tends to temporarily separate the conductors and extinguish the arc. Thus, parallel arcs are sometimes also referred to as sputtering arcs.
Series arcing involves an opening or disconnect in a wire or conductor carrying current which allows arcing to occur between one end of the disconnect to the other end. Series arc faults may thus exist when the arc occurs across a break in the line or neutral conductors or at a loose terminal in a branch circuit of a distribution network. The conductors are carrying current to a load derived from the line voltage. The arc could likewise occur as a break or at a loose terminal associated with an extension cord deriving power from line voltage, thereby completing the circuit to the load. Since the current through the series arc fault is limited by the impedance of the load itself, and since the fault is in series with the load, the fault is known as a series arc fault.
FIG. 1 shows how the current in a series arc circuit 110 must bridge or arc across an open path 118. Line and neutral conductors 114, 116 as well as the outer insulation 112, 119 are further shown in FIG. 1. The outer insulation 119 is shown as expanding, prior to rupturing, which may allow the arcing to contact external material which may cause damage.
Prior art methods and apparatus of detecting parallel arcing faults have proven successful. However, lower level arcs in series with a load may be beyond the capabilities of those techniques. Prior art circuit interrupters do not differentiate the frequency content change between switching loads such as drills, hair dryers, and air compressors and those with series arcs. Series arcing faults typically create currents with low root mean square (“RMS”) value, which is below the thermal thresholds for such circuit breakers. Even so, such arcs may cause damage or start a fire if they occur near combustible material. There are problems associated with merely increasing the sensitivity of prior techniques. For example, at increased sensitivities, there may be more loads and combinations of loads that can produce false trips.